EP1470263A1

EP1470263A1 - Method for depositing metal-free carbon layers

Info

Publication number: EP1470263A1
Application number: EP03718664A
Authority: EP
Inventors: Klaus Bewilogua; Ralf Wittorf; Helge Thomsen
Original assignee: Fraunhofer Gesellschaft zur Forderung der Angewandten Forschung eV
Current assignee: Fraunhofer Gesellschaft zur Forderung der Angewandten Forschung eV
Priority date: 2002-01-30
Filing date: 2003-01-28
Publication date: 2004-10-27
Anticipated expiration: 2023-01-28
Also published as: DE10203730B4; ATE442465T1; DE50311895D1; DE10203730A1; EP1470263B1; WO2003064720A1

Abstract

The invention relates to a method for depositing metal-free, amorphous, hydrogenous carbon layers, according to which substrates are sputtered by targets. A pure carbon target, especially a graphite target, is used as a target and is biased by a negative direct-current voltage. Preferably, the substrate is also supplied with a direct-current. The inventive method is preferably carried out using the unbalanced magnetron mode of operation, enabling metal-free DLC layers to be produced with a high wear rate and a high hardness value.

Description

Process for the deposition of metal-free carbon layers

The invention relates to a method for the deposition of metal-free, amorphous, hydrogen-containing carbon layers by sputtering targets onto substrates.

For many applications it is interesting to deposit relatively thin carbon layers on substrates. In particular, layers of hard and wear-resistant diamond-like carbon are involved, these layers also being referred to as DLC (diamond-like carbon) layers or also a-C: H layers. In addition to carbon, these layers also contain hydrogen with a concentration between 10 and 30 atom percent, metal components in less than 0.1 atom percent and other impurities (for example oxygen, nitrogen, argon) with a total concentration of less than 3 atom percent.

There are numerous publications on the deposition of these hard amorphous hydrogen-containing carbon layers (DLC or aC: H) and their properties. In addition to very low coefficients of friction compared to steel (<0.2), such layers have high hardness and wear resistance. The DLC layers are generally chemically inert, electrically insulating with resistivities of more than 10 ⁶ ohm centimeters and they are transparent in the infrared spectral range.

High-quality aC: H layers, especially high-quality in terms of hardness and wear, are still predominantly produced today in relatively small plants using plasma-activated chemical vapor deposition (plasma-activated chemical vapor deposition, or PACVD for short). High-frequency technologies with frequencies of more than one megahertz (MHz) are mainly used, usually 13.56 MHz. The plasma-generating electrode is also the substrate to be coated. Hydrocarbons, preferably acetylene (C ₂ H ₂ ) or methane (CH ₄ ), are used as the source of carbon. The layers are created by the deposition of ions and radicals from a glow discharge on an unheated substrate. It is very disadvantageous that these high-frequency methods for DLC deposition can only be scaled up with difficulty. A negative self-bias voltage U _B of a few 100 volts is required. This voltage U _B depends strongly on the ratio of the areas of the substrate electrode to the anode. The grounded housing of the system is generally used as the anode. U _B drops significantly with increasing size of the substrate electrode. A larger substrate area also requires more powerful high-frequency generators in order to maintain approximately constant power densities on the substrate. Such high-performance high-frequency generators are very expensive, however, and thus reduce the economic viability of such methods.

To avoid this problem, it has therefore already been proposed to use the medium frequency range between one and a few hundred kHz instead for the deposition of a-C: H layers. DE 195 13 614 C1 and others have proposed harmonic AC voltages as well as pulsed DC voltages. Here too, however, it is mostly a PACVD process, in which the substrate electrode is also the plasma-generating electrode. As a result, it is not possible to apply coating parameters such as, for example, the ion current and the ion energy. set independently. Here too, there are limits to the economic use of such processes.

Similar problems exist with conceptions, for example according to DE 196 51 615 C1, in which a PVD method is proposed. There, the target is operated with medium-frequency voltage in order to obtain a reactive sputtering process.

S. Fujimaki, H. Kashiwase and Y. Kokaku in "New DLC coating method using magnetron plasma in an unbalanced magnetic field" in: Vacuum 59 (2000), pages 657 to 664 suggested a modified procedure which they called Plasma Enhanced Chemical Vapor Deposition (PE-CVD) For coating magnetic storage disks, they are placed close to the target and in the discharge zone of the target as flat substrates in an area of high plasma density. The distance of the substrate from the target or the cathode is 40 mm and remains during the coating is constant since all elements to each other in a fixed, immovable position. An unbalanced magnetron cathode on the target provides plasma support. A pulsed direct voltage (DC) is present on the substrate in order to avoid substrate charging. The result is very thin (15 nm) and polymer-like layers with a high specific resistance but a relatively low hardness of less than 15 GPa. The extremely thin layer and the low hardness are not sufficient for many applications.

Older proposals, for example known from US Pat. No. 4,597,844, for the deposition of layers containing amorphous carbon and hydrogen use graphite targets and reactive gases containing only hydrogen and / or fluorine. This leads to polymer-like layers with very high resistivities of more than 10 ⁸ ohm centimeters and a hydrogen content in the layers of 0.5 to 0.9, measured by the ratio of the number of hydrogen atoms to the number of carbon atoms (H / C). These layers no longer meet the increased requirements.

For this reason, completely different approaches have been taken to achieve carbon layers with the appropriate properties. For example, metal-containing carbon layers have been proposed, for the first time in EP 0 087 836 B1. Such, mostly as Me-C: Marked H or Me-DLC layers are produced in indus- ^• centering-relevant dimensions in vacuum coating process, by sputtering (sputtering) of a solid surface (a target) by bombardment with energy-rich ions. Carbide-forming metals such as titanium, niobium, chromium, tungsten or also carbides such as tungsten carbide are usually used as target materials. A disadvantage of such layers, however, is that their lowest wear rates, measured with a calotte tester, are between approximately 2 and 5 VE, which means that they are significantly worse than those of hard a-C: H layers, which are approximately between 0.5 and 1 VE. 1 VE is a wear unit and corresponds to 10 ^"15 m ³ / Nm.

It has therefore been attempted to reduce the metal content in such layers by covering the metallic targets largely with carbon by means of a modified process control, which is referred to as "poisoning". However, such processes are difficult to master because the discharge processes become very unstable and cannot be maintained over long periods.

As a further approach, Da-Yung Wang, Chi-Lung Chang and Wei-Yu Ho in "Characterization of hydrogen-free diamond-iike carbon films deposited by pulsed plasma technology" in: Thin Solid Films 355 to 356 (1999), A coating process for a titanium-doped, that is, metal-containing, but hydrogen-free layer was proposed on pages 246 to 251. The authors attribute the lack of hardness of known layers to the presence of hydrogen atoms, both the graphite and titanium targets used and the substrate supplied with pulsed medium-frequency direct voltages (DC). The layers that arise are of course clearly different from hydrogen-containing coatings.

There is therefore still the desire to deposit metal-free carbon layers in order not to have to accept the disadvantages of the Me-C: H layers. A method for the deposition of such a metal-free carbon layer, which can also be used on a larger scale and economically, would be desirable.

The object of the invention is therefore to propose such a method.

This object is achieved by sputtering from targets onto the substrates or by means of arc evaporation, by biasing the targets with a negative DC voltage, by using a pure carbon target as a target, by biasing the substrate with a negative voltage and that an ion current density of at least 1 mA / cm ^{2 is achieved} on the substrate.

Surprisingly, it was found that with such a method the wear and hardness values known from conventionally produced aC: H or DLC layers can be achieved and in some cases even exceeded. Until now, it had not been thought possible to be able to achieve such values with a pure direct-current sputtering method, that is, without the use of high-frequency or at least medium-frequency methods. Only at Sputtering has so far been seen as meaningful for the poorer quality and very differently constructed Me-C: H layers. For aC: H layers, only discharges of carbon plasmas have been considered.

In addition, layers with hardnesses of more than 30Gpa and from 1 μm to several micrometers in thickness are produced, that is to say more than in the method of Fujimaki et al.

A distinction must be made between the negative bias on the target and the negative bias on the substrate. According to the invention, the negative bias voltage on the target itself is a negative DC voltage. In a preferred embodiment, the negative bias voltage on the substrate can and is really also a likewise negative DC voltage.

In another embodiment, however, it is also possible to excite the negative bias on the substrate with a medium frequency voltage between 30 and 300 kHz. It is then sputtered unchanged by the target with direct current, since there is a negative direct voltage, in contrast to DE 196 51 615 C1.

In DE 100 18 143 A1, a medium frequency is used on the substrate, but there is no sputtering from a target.

The distance of the substrate from the target is preferably greater than the discharge zone of the target. This can prevent the high plasma density frequently prevailing in the discharge zone of the target from causing a type of plasma-assisted CVD process to be carried out and contributing to polymer-like effects in the deposited hydrocarbon layers. The hardness of the layers is additionally supported by avoiding these effects.

Distances of the substrates from the target of more than 50 mm, in particular of more than 100 mm and particularly preferably of 200 mm and more, have been found to be favorable. A pure graphite target is preferably used as the carbon target, since this material has proven particularly useful. In certain applications, glassy carbon is also an advantage.

The best values can be achieved when working in the unbalanced magnetron mode. As a result, significantly higher ion current densities are achieved in a substrate than in balanced, conventional magnetron operation. These ion current densities have been found necessary to separate the hard DLC modification.

The method according to the invention for deposition can be scaled up relatively easily, as can the method with which Me-DLC layers, that is to say significantly poorer layers, can otherwise be scaled up.

The method according to the invention is a pure direct current method. As a result, no cost-intensive high-frequency (HF) or medium-frequency (MF) power supplies are required, and there are also no plasma states that are difficult to control, and therefore no discharges at undesirable locations in the coating chamber that occasionally occur at medium-frequency and high-frequency.

According to the invention, a coating quality can be achieved more reliably, more cost-effectively and also with less expenditure on equipment, which is readily comparable to the conventional one which can be achieved with much greater expenditure and can also be applied to significantly larger substrate areas.

It is also particularly advantageous that the method according to the invention for the deposition of metal-free carbon layers can also be integrated into a PVD (physical vapor deposition) coating arrangement. This makes it particularly easy to first separate the intermediate layers made of transition metals such as titanium, niobium or chromium, which seem sensible for optimal adhesion, before the actually desired metal-free carbon layer is then deposited over this intermediate layer. The coating method according to the invention is extremely flexible. When using multiple targets, DLC or Me-DLC layers can even be deposited with a system suitable for the process, even in multilayers.

In addition, combinations of, for example, DLC and carbon nitride layers (CN _X layers) can be produced. For this purpose, the hydrocarbon reactive gas is replaced by nitrogen, for example, at the end of the coating process. Such a layer is useful, for example, if hard and also good wetting layers are required. CN _X (X to 0.2) has a significantly higher surface energy and thus better wetting than DLC.

A mixture of one or more noble gases and one or more hydrocarbons has proven itself as the reaction gas. A mixture of argon and acetylene is particularly preferred.

To improve the adhesion, for example, intermediate layers can be deposited according to the invention, which consist of metals, metal nitrides, metal carbides, metal carbonitrides or also multilayer arrangements of these components. A thickness range between 0.02 and 2 μm is particularly preferred.

In addition, a cover layer is preferably deposited which contains only carbon and hydrogen and / or nitrogen and less than 0.1% metal impurities.

In one embodiment of the invention, the reactive gas is mixed during the coating process with additional gaseous or vaporous compounds and with nitrogen and / or oxygen or replaced by nitrogen.

It is preferred if the additional gaseous or vaporous compounds contain silicon or if the additional gaseous or vaporous compounds contain tetramethylsilane (TMS) or if the additional gaseous or vaporous compounds contain hexamethyldisiioxane (HMDSO). Silicon is not a metal, even if it has metallic properties. The installation silicon in the desired layers is particularly easily possible with these additional gaseous or vaporous compounds.

In the following, possibilities for carrying out the method according to the invention are described using some exemplary embodiments. These are practically proven design examples.

The exemplary embodiments have been carried out in a large coating chamber which is particularly useful for carrying out a scaled-up coating. The substrates to be coated can be rotated and moved through this coating chamber, for example with a double rotation. The diameter of the substrate holder was 600 mm when the exemplary embodiments were carried out.

Due to the mobility of the substrates, the distance of the substrate from the target was variable. The minimum distance was 200 mm, but the distance was also up to about 500 mm. The mobility of the substrates can of course also be used well for series production.

Due to the relatively high distances of the substrates from the cathode, it can be ensured that the substrates are located outside the discharge zone of the target. This reliably prevents the substrates from getting into the discharge zone of the target, which often has a high plasma density that is otherwise used for a plasma-assisted CVD process. However, this effect would be less desirable here, particularly in the case of the particularly hard layers to be produced. Rather, in the design of the exemplary embodiments, the substrates are subject to the sputter particle stream from the target, which consists of dusted carbon and hydrogen.

In a first example, a metal-free carbon layer is deposited in a direct current magnetron sputtering system with four sputtering cathodes. Unbalanced magnetrons are used. Flat samples made of 100 Cr6 ball bearing steel and thin, 0.2 mm silicon disks, HSS twist drills and other metallic substrates with complex geometries are used as substrates. The substrates are placed on standard substrate holders Multiple rotations (2-fold or 3-fold) fastened and rotate during the entire coating process.

A total of four targets are used, namely two made of graphite and two made of titanium. The latter serve to produce a metallic adhesion promoter layer which is to be arranged below the metal-free carbon layer.

The coating process can be divided into four sections: in a first section, the substrate is cleaned by ion etching (Ar ⁺ ), the targets being covered by a shutter, the so-called shutter. In a second step, this shutter is opened in front of the two titanium targets, while the two graphite targets remain covered. The metallic adhesive layer made of titanium is applied by non-reactive sputtering of the titanium with argon.

In a third section the reactive sputtering process is started. The reactive gas C ₂ H ₂ is first slowly added to the sputtering gas and the C ₂ H ₂ gas flow is increased in a ramp up to the specified final value. The titanium targets are switched off and the graphite targets start operating by opening the corresponding shutters.

In a fourth step, the metal-free carbon layer is deposited in the form of a DLC (diamond like carbon) layer with a constant C ₂ H ₂ flow at a constant direct current bias voltage of minus 300 volts. In a practical test, the deposition rate is 0.8 μm per hour and the layer thickness achieved is 2.5 μm.

The layer composition can be analyzed by means of secondary ion mass spectroscopy (SIMS) and the structure by means of Raman spectroscopy. To characterize the mechanical and tribological properties, the hardness and abrasive wear and also the adhesion are interesting. The hardness can be determined with an Indentor method, the wear values with a calotte tester and the adhesion with a Rockwell indentation test as well as with a scratch test. A test evaluation shows that both the composition of the layers as well as the wear and hardness values and the adhesion of the layer produced according to the invention are practically identical to those which could be achieved with a conventional DLC layer deposited by means of high-frequency methods. Despite the significantly more advantageous and simpler manufacturing options, there is no quality night in the layer.

The Rockwell hardness (HRC) of the substrates is almost unchanged in the conventional as well as in the method according to the invention compared to the state before the coatings (HRC approximately 62). This shows that the substrate temperatures in the conventional method and in the method according to the invention each remain below 200 ° C.

In a second example, the layer is also deposited in a direct current magnetron sputter system with four sputter cathodes. Again, unbalanced magnetrons are used. Two targets consist of graphite, the other two targets of niobium (Nb). The substrates are the same as in the first example.

The coating process again runs in four sections. First, substrate cleaning is carried out by ion etching (Ar ⁺ ), the targets being covered by an aperture (shutter). Secondly, the shutters are opened in front of the niobium targets and the metallic adhesive layer (Nb) is applied by nonreactive sputtering of niobium with argon. The third step is to switch to the reactive sputtering process by means of the two graphite targets, namely by slowly adding the reactive gas C ₃ H ₈ to the sputtering gas and by switching off the niobium targets. The C ₃ H ₈ gas flow is increased in a ramp up to the specified final value. Finally, the carbon layer is again deposited as a DLC layer with a constant C ₃ H ₈ flow at a constant direct current bias voltage of minus 200 volts. The deposition rate in this test is 1.4 μm per hour and the layer thickness achieved is 3 μm. An analysis of the layers gives the same results as in the first example.

In a third example, in contrast to the two previous examples, flat samples made of different types of steel are coated. The rotation is omitted here. With a substrate arrangement directly opposite a graphite target, a coating rate that is approximately three times higher is achieved.

The metallic interlayer is first deposited in front of a titanium target. The substrates are then pivoted in front of a graphite target and then remain in this position. Then, as in the other examples, the steps of transferring to the reactive sputtering process and deposition of the carbon layer are carried out. In this example, acetylene is used as the carbon carrier.

The deposition rate is 2.3 μm per hour, the layer thickness achieved is 5 μm. The Rockwell hardness (HRC) of the substrates after the coatings are also almost unchanged compared to the state before the coatings with HRC approximately 62, even when this example according to the invention is compared with a conventional method. Here, too, the substrate temperatures have remained below 200 ° C.

The following table summarizes some of the results found in the actual examples.

Claims

Expectations

1. A process for the deposition of metal-free amorphous hydrogen-containing carbon layers on substrates, characterized in that targets are sputtered onto the substrates or deposited by means of arc evaporation, that the targets are biased with a negative direct voltage that a pure carbon target is used as the target that the substrate is biased with a negative voltage and that an ion current density of at least 1 mA / cm ^{2 is achieved} on the substrate.

2. The method according to claim 1, characterized in that the distance of the substrate from the target is greater than the discharge zone of the target.

3. The method according to any one of claims 1 or 2, characterized in that the distance of the substrate from the target is greater than 50 mm, in particular greater than 100 mm, preferably greater than 200 mm.

4. The method according to any one of claims 1 to 3, characterized in that a graphite target is used as the target.

5. The method according to any one of claims 1 to 3, characterized in that a glassy carbon target is used as the target.

6. The method according to any one of the preceding claims, characterized in that several targets are used.

7. The method according to claim 6, characterized in that different targets which can be separated by diaphragms are used, which are released at different times and bring about a double or multilayer coating of the substrate.

8. The method according to claim 7, characterized in that at least one of the further targets is a metallic target.

9. The method according to claim 7 or 8, characterized in that an intermediate layer for improving the adhesion of metals, metal nitrides, metal carbides, metal carbonitrides or multilayer arrangements is deposited with these components, preferably with a thickness between 0.02 μm and 2 μm.

10. The method according to any one of claims 7 to 9, characterized in that a cover layer of carbon, hydrogen and / or nitrogen with metal impurities of less than 0.1% is deposited.

11. The method according to any one of the preceding claims, characterized in that the substrate is biased with a negative DC voltage.

12. The method according to any one of the preceding claims, characterized in that the substrate with a medium frequency voltage between 30 and

300 kHz is excited.

13. The method according to any one of the preceding claims, characterized in that the high ion current density on the substrate is achieved by working in the unbalanced magnetron mode.

14. The method according to any one of the preceding claims, characterized in that a mixture of noble gases and hydrocarbon gases and / or hydrocarbon vapors is used as the reactive gas.

15. The method according to claim 14, characterized in that a mixture of argon and acetylene is used as the reactive gas.

16. The method according to any one of the preceding claims, characterized in that the reactive gas is mixed during the coating process with additional gaseous or vaporous compounds and with nitrogen and / or oxygen or is replaced by nitrogen.

17. The method according to claim 16, characterized in that the additional gaseous or vaporous compounds contain silicon.

18. The method according to claim 16 or 17, characterized in that the additional gaseous or vaporous compounds contain tetramethylsilane (TMS).

9. The method according to any one of claims 16 to 18, characterized in that the additional gaseous or vaporous compounds contain hexamethyl disiloxane (HMDSO).